The Fischer-Tropsch technology is known in literature, for preparing hydrocarbons from mixtures of gas based on hydrogen and carbon monoxide, conventionally known as synthesis gas. A compendium which summarizes the main works on the Fischer-Tropsch synthesis reaction is contained in the Bureau of Mines Bulletin, 544, (1955) entitled “Bibliography of the Fischer-Tropsch Synthesis and Related Processes” H. C. Anderson, J. L. Wiley and A. Newell.
The Fischer-Tropsch technology is generally based on the use of slurry reactors, reactors which are normally used in relation to chemical reactions which are carried out in three-phase systems in which a gaseous phase is bubbled into a suspension of a solid in a liquid. The gaseous phase consists of synthesis gas, with a molar ratio H2/CO ranging from 1 to 3, the dispersing liquid phase at a high temperature, represents the reaction product, or mainly linear hydrocarbons with a high number of carbon atoms, and the solid phase is represented by the catalyst.
The reaction product which is discharged from the reactor therefore consists of a suspension which must be treated to separate the solid (catalyst) from the liquid phase. Whereas the catalyst is recycled to the synthesis reactor, the liquid is subjected to subsequent treatment, for example hydrocracking and/or hydro-isomerization treatment, to obtain hydrocarbon fractions of industrial interest.
European patent 609,079 describes a reactor for Fischer-Tropsch reactions consisting of a gas-bubbling tower containing a suspension consisting of particles of catalyst suspended in the liquid hydrocarbon. The synthesis gas is fed to the base of the reactor whereas the synthesized hydrocarbon is recovered at the head.
In order to avoid the entrainment of catalyst particles, the reactor is equipped with cylindrical filtration devices internally arranged in its upper part.
International patent application WO 97/31693 describes a method for separating a liquid from a suspension of solid particles which comprises, in a first phase, degasifying the suspension and, in a second phase, filtering the suspension through a tangential flow. In particular, the suspension comes from a Fischer-Tropsch reactor and consists of synthesized heavy hydrocarbons which entrain the catalyst particles.
Other examples of methods for separating the catalyst contained in the suspension leaving a Fischer-Tropsch reactor are described in European patent 592,176, international patent application WO 94/16807, English patent 2,281,224, U.S. Pat. Nos. 4,605,678 and 5,324,335 and German patent 3,245,318.
A disadvantage associated with Fischer-Tropsch processes, for example with those mentioned above, and in particular Fischer-Tropsch processes in which the catalyst is based on cobalt, is that a liquid hydrocarbon is produced, which, in the subsequent transformation phases (hydroisomerization and/or hydrocracking), causes an alteration in the performances of the corresponding catalysts.